1. Field of the Invention
The present invention relates to a yoke for universal joint and a production process thereof and, more particularly, to a yoke for forming a universal joint which connects ends of a pair of rotary shafts not existing on one straight line with each other so as to enable transmission of rotational force between these two rotary shafts.
2. Related Background Art
A steering apparatus for an automobile is constructed for example by serially connecting a plurality of rotary shafts including a steering shaft, intermediate shafts, and the like through a universal joint. Universal joints (and Cardan joints,) in each of which a cross shaft interconnects a pair of yokes so as to allow displacement thereof, have been used widely. It has also been widely practiced to make the yoke forming such a universal joint by plastic working of a metal sheet with sufficient stiffness, such as a steel sheet.
FIGS. 11A-11C and FIGS. 15A-15C show examples of yokes for universal joint made in this way. Each of yoke 1 and yoke 1c is composed of a base 2 and a pair of arms 3, 3 or 41, 41 extending from one axial edge of this base 2. In an area apart from the arms, the base 2 is formed in an incomplete cylindrical shape discontinuous at one position in the circumferential direction, for insertion of an end of a rotary shaft to which the yoke 1 is to be fixed. A pair of flanges 5, 6 opposite to each other are provided at the discontinuous part. A through hole 18 for a stem part of a bolt (not illustrated) to be inserted therein is formed in one flange 6, while a screw hole 8 coaxial with the through hole 18 is formed in the other flange 5 so that the stem part of the bolt may be coupled therewith.
The above arms 3, 3 or 41, 41 extend in the axial direction of the base 2 from opposite positions in the diameter direction and on one axial edge of the base 2. These arms 3, 3 or 41, 41 have mutually opposite surfaces, which are cylindrical, concave surfaces (FIGS. 11A-11C) or flat surfaces (FIGS. 15A-15C). Further, mutually coaxial, circular holes 4, 4 or 42, 42 are formed in the tip portions of the respective arms 3, 3 or 41, 41. For constructing a universal joint, the above yoke 1 is combined with a cross shaft 9; bearing cups 10, 10, which are pressed into the associated circular holes 4, 4 or 42, 42. Each of these bearing cups 10, 10 functions as an outer ring of a radial needle bearing and couples the yoke 1 or yoke 1c with the cross shaft 9 to support the cross shaft 9 so as to be rockable relative to the yoke.
The yoke 1 or yoke 1c as described above has been produced heretofore in the production process as shown in FIGS. 12A-12D. First, a metal sheet with sufficient stiffness, such as a steel sheet, is punched by press working to obtain a flat blank sheet 11 having the shape as shown in FIG. 12A. This blank sheet 11 has a nearly rectangular base portion 12 and a pair of flaps 13, 13 projecting in a rabbit ears shape from one edge of the base portion 12. This blank sheet 11 is pressed between a pair of press dies to be subject to plastic deformation into the shape as shown in FIG. 12B, thus obtaining a first intermediate blank 14. This first intermediate blank 14 is shaped so that each of the portions of flaps 13, 13, which are to become a pair of arms 3, 3 or 41, 41 (FIGS. 11A-11C or FIGS. 15A-15C), is curved in a partially cylindrical shape or is flat and so that a projecting portion 15 is provided at a portion which is a part of the base portion 12 to become the base 2 and where the screw hole 8 (FIG. 11B or FIG. 15B) is to be formed. Then this first intermediate blank 14 is curved to form the central part of the base portion 12 in the incomplete cylindrical shape as shown in FIGS. 12C and 12D, thereby obtaining a second intermediate blank 16.
In the second intermediate blank 16 constructed in the shape as shown in FIG. 12D in this way, serrations 17 are cut in the internal periphery of the portion corresponding to the base 2, and the through hole 7 and screw hole 8 (FIGS. 11A-11C or FIGS. 15A-15C) are formed respectively in the portions corresponding to the flanges 5, 6. Further, the circular holes 4, 4 or 42, 42 (FIGS. 11A-11C or FIGS. 15A-15C) are formed in the associated tip portions of the portions corresponding to the respective flaps 13, 13. For forming these circular holes 4, 4 or 42, 42, the conventional process is arranged to include steps of forming a prepared hole of a small diameter by a drill, thereafter enlarging the inner diameter of this prepared hole by an end mill, and further finishing the inner edge of this hole by a reamer, thereby obtaining each circular hole 4, 4 or 42, 42.
The well known yokes for universal joint with the cross shaft include not only those in the structures as shown in FIGS. 11A-11C and FIGS. 15A-15C, but also those in the structures as shown having FIGS. 13A-13C and FIGS. 14A-14C. First, the yoke 1a in the second example shown in FIGS. 13A-13C has the base 2a formed in a U-shaped cross section. The rotary shaft to be coupled with the end of such yoke 1a is formed in an oblong shape of cross section at least at the end thereof, thereby preventing the rotary shaft from rotating relative to the yoke 1a when they are coupled with each other. In the case of the structure shown in FIGS. 13A-13C, a nut 19 is pressed and fixed in the through hole 7 formed in the flange 5, thereby forming a screw hole for a bolt to be coupled therewith. Further, the yoke 1b in the third example shown in FIGS. 14A-14C has the base 2b formed in a cylindrical shape. For coupling such yoke 1b with the end of rotary shaft, the end of the rotary shaft is pressed into engagement with the base 2b by close fit. Formed in the tip portion of each arm 3, 3 forming the yoke of the second or third example is the circular hole 4, 4 for engaging with the bearing cup 10 (FIG. 11A) and fixing it, similarly as in the yoke 1 of the first example shown in FIGS. 11A-11C described above. These circular holes 4, 4 are also finished in the predetermined inner diameter by using the drill, end mill, and reamer in order, similarly as in the case of the yoke 1 shown in FIGS. 11A-11C above.
In the cases of the yokes 1, 1a, 1b, and 1c known conventionally, because the circular holes 4, 4 or 42, 42 were formed in the tip portion of each arm 3, 3 or 41, 41 by using the drill, end mill, and reamer in order, processing of the circular holes 4, 4 or 42, 42 was cumbersome and raise the production cost of yoke 1, 1a, 1b, or 1c.
It has been considered to employ a press for processing of circular holes 4, 4 or 42, 42 for insertion of the cross shaft 9. However, as a practical matter, processing of the holes by press has not been feasible due to poor accuracy resulting from the following factors.
(1) When the cross section of arm 3, 3 is curved, it is not easy to match punch and die for piercing closely with the curved portion of arm 3, 3. PA1 (2) Since the width is narrow of a bridge at the tip portion of each arm 3, 3 or 41, 41, the bridge part fails to resist a processing load upon press piercing so as to be crushed.
Therefore, the press has been used at most for processing of an unfinished hole, so that final machining was indispensable.